1. Field of the Invention
The present invention relates to an optical information recording medium, and particularly to a re-writable optical information recording medium, which is recordable with heat mode.
2. Description of the Related Art
With the start of BS digital broadcasting with an image quality of high definition television (HDTV) approaching, development of a high-capacity information recording medium, which is capable of recording the BS digital broadcasting of HDTV quality for about two hours, is now under way. In an optical information recording medium for a blue-violet laser light source, which recording medium is expected to be prevalent as a next-generation optical disk, a recording layer formed on a substrate is covered with a thin cover layer having a thickness of 0.1 to 0.3 mm. In a system employing such an optical information recording medium, a pickup employs an objective lens having a high numerical aperture (NA) to focus a laser beam and the recording is performed by irradiating the optical disk from the thin cover layer side thereof with the laser beam. Thus a high recording capacity per-side of 22.5 Gbyte is being realized.
The recording layer of the optical information recording medium is covered with a cover layer as explained above, and the cover layer is provided on the recording layer via an adhesive. Examples of methods for providing the cover layer generally include, for example, a method of spin coating a substrate with an ultraviolet-curable adhesive and sticking the cover layer thereon, and a method of coating a substrate with an adhesive to provide the cover layer thereabove.
Among these two methods, the method utilizing the coating of the adhesive is advantageous because it forms optical information disks having a smaller fluctuation of the thickness between the internal periphery and the external periphery but results in a higher level of noises, in comparison with the method of spin coating a substrate with the ultraviolet-curable adhesive and sticking the cover layer thereon.